Xous

Embedded Multiprocess Microkernel

17 April 2024, Sean Cross

Outline

  1. About Me
  2. Project history
  3. Why a new OS?
  4. What can it do?
  5. How do I use it?

About Me

  • Live in Singapore
  • Enjoy understanding systems
  • Low-level is best

Novena: Open laptop

  • Always wanted custom laptop
  • "Relatively Large" Cortex A9
  • 32 GB disk, 4 GB RAM
  • Linux

Chibitronics: Programming for 8-year-olds

  • Paper circuit design
  • "Relatively Small" Cortex M0+
  • 32 kB flash, 4 kB RAM
  • ChibiOS

Precursor Design

  • Two FPGA-based CPUs
  • RV32IMAC based on VexRiscv
  • 128 MB flash, 16 MB RAM
  • Operating System?

Operating System Selection

  • RISC-V
  • Memory Protection
  • Not Linux
  • Rust is a new thing?

Xous Design

Brief Specs

  • Kernel is 17 ksloc
  • Threads
  • Processes
  • Tier 2 Rust support
  • Entire project built on stable Rust

Other Features

  • Microkernel
  • Interrupts in userspace
  • Each page of memory is mapped once
  • No C compiler required

More Specs

  • Kernel is single-threaded
  • Services use well-defined API calls
  • All drivers provided in user space
  • Each page of memory can only be lent once

Messages are Everything

  • Messages form IPC
  • Scalars or Pages
  • Blocking or Nonblocking

Example: Sleeping

sleep(msec: usize)
  1. Connect to "ticktimer" server
  2. Send a `BlockingScalar` with desired duration
  3. Client will block until server replies
  4. Server calls `ReturnScalar()` after expiration

Example: Mutex

mutex.lock().unwrap()
  1. Fast path with AtomicIsize
  2. Slow path by sending LockMutex
  3. On unlock, contending thread will see poison and send UnlockMutex
  4. Server will reply to LockMutex message to unblock the thread

MMUs are Amazing

  • XIP flash for free!
  • Swap space
  • Encrypted swap? No problem!
  • Realtime patching broken hardware

How can I use it?

  • Talk to system vendors -- get them to include MMUs!
  • Emulation is easiest -- Renode support

Thank you!

github.com/betrusted-io/xous-core